Well known in the art is a so-called twin roll continuous casting apparatus in which a pair of internally cooled rolls having respective horizontal axes and rotating in opposite direction to each other are disposed parallel to each other with an appropriate gap therebetween, a pool of molten metal is formed on the circumferential surfaces (the upper halves of cylindrical surfaces in the axial directions) of the rolls above the gap and the molten metal is continuously cast into a metal strip through the gap while being cooled by the circumferential surfaces of the rotating rolls. There has also been proposed such a twin roll continuous apparatus applied to a case of continuous casting of steel to produce a steel strip directly from molten steel.
When a metal strip is continuously cast through a gap between a pair of rolls, it is necessary to form a pool of molten metal on the circumferential surfaces of the pair of rolls above the gap therebetween and to maintain a level of the molten metal in the pool substantially constant by continuously pouring the molten metal into the pool. In order to form the pool of molten metal, there are required a pair of dams having their surfaces perpendicular to the roll axes which prevent an overflow of molten metal along the roll axes on the circumferential surfaces of the rolls. These dams also serve usually to regulate the width of the cast strip and are referred to herein as "side dams". In addition to the side dams disposed at the left and right sides of the rolls, a pair of front and rear gates having their surfaces along the roll axes may be erected orthogonally to the side dams on the circumferential surfaces of the rolls so as to form a box-like pool for molten metal with the side dams and the front and rear gates. However, when the pair of rolls have sufficiently large radii respectively, the front and rear gates along the roll axes are not always needed. In this case, the circumferential surfaces of the pair of rolls may fulfill by themselves roles of the front and rear gates.
There are known, as the pair of side dams, movable side dams which urge a pair of endless metal belts, caterpillars and the like against both edge surfaces of the rolls (side surfaces of the rolls perpendicular to the roll axes) at a location of the roll gap and move at a speed corresponding to the casting speed, and fixed side dams which have plate-like bodies of refractories fixed to left and right side surfces of the rolls. Generally, with the latter fixed side dams, the constitution of the apparatus is simple and the control of running is not complicated, compared with the former movable side dams.
Two systems of the fixed side dams are known. One is a system in which the distance between the plate-like bodies of the fixed side dams is smaller than the roll width (the length of roll from one end to the other end), and the other is a system in which the distance is the same as the roll width. According to the former system, the pair of side dams are erected on the circumferential surfaces of the rolls such that the bottoms of the side dams slidably contact the circumferential surfaces of the rolls. According to the latter system, the side dams are fixedly provided so that the respective inside surfaces of the side dams slidably contact the side surfaces of the rolls, that is, the pair of side dams sandwich the pair of rolls on the side surfaces of the rolls.
Usually, the fixed side dams are made of refractory material having a good adiabatic property. This is because the molten metal contacting the side dams has to be prevented from being solidified on the surfaces of the side dams. Adiabatic refractory materials generally have inferior wear resistance to that of solidified metal and liable to have scratches. Thus, the fixed refractory side dams may be damaged during the running of the apparatus, and the increase of damages may bring about break-out of molten metal. Further, according to the system noted above in which the side dams are fixed so that they sandwich the rolls on their side surfaces, clearances may be formed between the side surfaces of the rolls and the inside surfaces of the side dams slidably contacting therewith due to pressure of the ends of the strip being cast applied at the time of passing through the roll gap, and the molten metal may enter the clearances. If such troubles occur, stable casting may no longer be continued. Accordingly, it has generally been considered that refractory materials suitable for the side dams should have a good wear resistance and the highest possible strength.
During the continuous casting, a portion of molten metal in the pool forms thin solidified shells respectively on the surfaces of the rotating rolls, and then these shells pass through the gap between the twin rolls while growing along with rotation of the rolls. At this time, the solidified shells are depressed (rolled) at a portion in the neighborhood of the smallest gap between the rolls to form into a metal strip of a predetermined thickness. Thus, owing to this depression (rolling), the solidified shells tend to expand widthwise near the roll gap. As a result, the ends of the cast strip apply large pressure to the side dams. In the case of the movable side dams wherein the side dams are moved at a speed corresponding to the casting speed, a problem of friction between the side dams and the ends of of the cast strip is not substantially posed. In the case of the fixed side dams, however, large friction is inevitably generated between the ends of the moving cast strip and the fixed side dams, and can be a cause of damages of the refractory side dams, occurrence of cracking and undesirable deformation of the ends of the cast strip, formation of clearances between the side surfaces of the rolls and the inside surfaces of the side dams slidably contacting therewith, and entrance of molten metal into the clearances so formed, all of which hinder stable continuous casting. These problems are especially serious in the case of continuous casting of steel wherein the material involved is higher melting and has higher strength, when compared with cases wherein lower melting and mild non-ferrous metals are concerned.
In Japanese Patent Application No. 62-84,555 (published as JP A-63-252,646 on Oct. 19, 1988, after the priority date of the present international application, that is, July 22, 1988; the corresponding U.S. patent application was issued as U.S. Pat. No. 4,811,780 on Mar. 14, 1989.), we have proposed a continuous casting apparatus for metal strip which may be said "abradable dam system" or "semi-movable dam system" intermediate between "movable" and "fixed" dam systems. According to our prior proposal. a refractory material capable of being well abraded is used as the material for the side dams, contrary to the prior art concept that refractory materials suitable for the side dams should have a good wear resistance and the highest possible strength. The abradable side dams are forcibly fed or moved in the casting direction during the casting while being frictionally abraded by slidably contacting surfaces of the rotating rolls and ends of the strip being cast. Repeated runs of continuous casting by the abradable dam system have indicated that further improvements are desired for a further stable running of continuous casting.
In Japanese Patent Application No. 61-21,4853 (published as JP A-62-214,863 before the priority date of the present international application; the corresponding U.S. patent application was issued as U.S. Pat. No. 4,754,802 on July 5, 1988.), we proposed a continuous casting apparatus for metal strip in which a pair of side dams are used, each side dam comprising a combination of a fixed dam which is not abradable) and a movable dam which is a movable belt. According to this prior art, the fixed side dams of a non-abradable refractory material are disposed above the narrowest position of the rolls, and the movable side dams are disposed below the fixed side dams. Our later experiences have revealed that the above-discussed problem of damages of fixed dams is also associated with this system of a combination of fixed dams and movable dams.